To apply breath-supporting respiration, pressure- or volume-controlled respirators are known, which make possible a plurality of respiration functions during the inspiration phases and expiration phases.
In prior-art respirators, the inspiration valve is closed and the expiration valve is opened at the beginning of expiration. The expiration valve is then actuated such that a predetermined end-expiratory pressure becomes established in the expiration line. This pressure is usually measured near the end of the expiration phase, when the expiration gas flow has reached a low value. Based on inevitable tolerances, the pressure actually measured always deviates from the preset value, and one seeks to actuate the expiration valve such that the deviation between the measured and preset pressures will be as little as possible. This pressure, hereinafter called the closing pressure of the expiration valve, can only be measured accurately as long as a small gas flow is flowing over the expiration valve.
Respirators also have mechanisms, hereinafter called “triggers,” for recognizing the patient's inspiratory efforts in order to trigger mechanical breathing support. The trigger is an important circuit element of a respirator and is a prerequisite for the patient to be able to communicate the initial pulse for the mechanical respiration stroke to the respirator. The trigger can be triggered by a pressure drop in the expiration line, which is generated by the patient's inspiration effort. The trigger threshold must be set such that it is below the end-expiratory pressure level. However, on the other hand, it also must not be so low that the patient would have to exert an excessively great inspiratory effort.
A continuous flow rate from the expiration valve is set in a respirator known from EP 459 647 B1. The gas flow is measured in the inspiration line and in the expiration line by means of two gas flow sensors, and the difference is determined. When the patient begins to breathe, part of the gas flow enters the patient's lungs and the gas flow in the expiration line is correspondingly reduced. If the difference of the gas flow exceeds a certain threshold value, the expiration valve is closed by a control unit of the respirator and an inspiration stroke is triggered. The pre-inspiratory continuous flow rate is restored at the end of the expiration.
To prevent the pressure from dropping during the patient's inspiratory effort in the prior-art respirator, the continuous gas flow must be distinctly higher than the patient's inspiratory gas flow to be recognized. Since the gas flows are measured with two different sensors, the measuring uncertainties of the sensors limit the sensitivity to recognize an inspiratory effort. High continuous gas flow rate also impairs the determination of the patient's respiratory minute volume, because the inspiration stroke applied must be determined from the difference of the gas flow in the inspiration line and that in the expiration line. The sensitivity of triggering is therefore not so high as it would be necessary for breathing support for critically ill patients.